Methods of preparing stannous pyrophosphate



trite States Patent 3,028,216 METHODS OF PREPARING STANNOUSPYROPHOSPHATE Robert J. Gemmell, Brentwood, and George Douglas Nelson,St. Louis, Mo., assignors to Monsanto Chemical Company, St. Louis, Mo.,a corporation of Delaware No Drawing. Filed May 25, 1956, Ser. No.587,195 2 Claims. (Cl. 23-105) This invention relates to methods formaking stannous pyrophosphate. Stannous pyrophosphate has been found tobe suitable for use as a dentifrice polishing agent, and when producedin accordance with this-invention pro-. vides a dentifrice basecharacterized by desirable properties such as opacity, freedom from agritty feeling in the mouth, low glycerine absorption, and suitablepolishing power. I a e It is an object of the invention to providemethods for preparation of stannous pyrophosphate which are superior topreviously-known methods andlwhich' produce 'a pyrophosphate having theabove desirable properties.

It is now well known that the application of certain soluble fluoridesin restricted amounts to the teeth pie: vents or retards the developmentof dental caries. One way in which the fluorides are applied to theteeth is by incorporation into dentifrice cleansing and polishingcompositions, such as tooth pastes and tooth powders. Tooth pastes andpowders contain substantial amounts of polishing agents. These polishingagents must be carefully selected and controlled from the point of viewof hardness, so that normal use of such pastes or powders will not causeundue tooth injury by severe abrasion.

Various phosphates, such as orthophosphates, are presently widely usedas polishing agents in dentifrice cleansing compositions. However, if anorthophosphate, such as dicalcium orthophosphate, is utilized in adentifrice composition containing a soluble fluoride salt, the fluorideanion will react with the orthophosphate to form an insolublefluoropatite and/or calcium fluoride, thereby destroying theavailability and effectiveness of the fluoride for the prevention ofdental caries.

When certain soluble fluoride salts such as tin fluoride are used indentifrice formulations, it has been found that the metallic "cation hasa synergistic effect on the action of the fluoride in inhibiting toothdecay. However, when an orthophosphate is used as a polishing agent, themetallic cation is precipitated as an insoluble metal phosphate. By useof the stannous pyrophosphate of the present invention, both of theabove types of precipitation are markedly retarded in fluoridedentifrices.

Stannous pyrophosphate may be used as a dentifrice base, alone or withother fluoride compatible polishing agents, such as calciumpyrophosphate or insoluble sodium metaphosphate. The said group ofcompounds provides for the presence of pyrophosphate in a form whichavoids the above-described disadvantage of the prior art compounds. Atthe same time stannous pyrophosphate is characterized by low abrasion,which is an important property in the formulated dentifrice.

Stannous pyrophosphate prepared in accordance with the present inventionis essentially insoluble in water, and is characterized by the presenceof little or no orthophosphate. The method of preparation of the presentinvention is the reaction of sodium acid pyrophosphate with stannouschloride. The reaction is conducted in an aqueous medium, so that it isdesirable to provide solutions, slurries or suspensions of the startingmaterials in water.

In accordance with the present invention, there is added to a slurry oraqueous solution of stannous chloride a solution of sodium acidpyrophosphate. The stannous pyrophosphate is reacted in stoichiometricproportion, although an excess thereof may be employed if desired. Thereaction is conducted in a mixing vessel provided with agitation meanssuch as a shaft stirrer or propeller. A preferred temperature range isfrom 35 C. to 100 C., which is maintained until conversion issubstantially complete in the production of the insoluble stannouspyrophosphate. In order to achieve complete precipitation it isdesirable to neutralize the solution with a base such as NaOH or Na COto produce normal pyrophosphate salts. The metal pyrophosphate thusobtained may be the anhydrous form, particularly as the result ofprolonged heating. A preferred reaction time is from 0.5 to 30 hours.The reaction mixture containing a slurry of the insoluble stannouspyrophosphate and soluble sodium salts is then filtered to remove thesoluble salts and to obtain the desired stannous pyrophosphate as theresidual product. The dry product is ground to the desired fineness fordentifrice purposes, such as 375 mesh particles.

The hardness, polishing power or abrasion of the stannous pyrophosphateof the present invention may be expressed in terms of a convenientnumerical scale as described below. The pyrophosphate polishing agent,which has been the preferred polishing agent in the past, i.e., calciumpyrophosphate, generally had a polishing power of between 30 and about40, although in a few isolated instances, material might have beenobtained having a polishing power as low as about 25. In contrastthereto, the pyrophosphate of the present invention can have a polishingpower of about 5. The range of polishing power or abrasion values whichmay be obtained generally run between about 3 and 30. The preferredrange of hardness or polishing power of the said pyrophosphate for usein fluorine-containing dentifrice compositions is between about 10 andabout 25.

The following method is used for determining the polishing power of thepyrophosphate produced in accordance with the present invention.

The method is based on the weight of metal abraded from an electrolyticcopper strip by brushing with a tooth brush having natural bristlesunder standard conditions in a glycerine suspension of the abrasive tobe tested. A clean copper strip is rinsed with alcohol, dried in an ovenat C., cooled in a desiccator, weighed, and inserted into a suitableholder, which is located in a trough. The tooth brush is attached to areciprocating holder, moistened, and then lowered against the copperstrip, the brush being adjusted so that it touches the copper stripalong the entire length of the brush. Once this has been achieved, thebrush is rigidly fixed in position. The sample to be tested is preparedby weighing 20 g. of the powdered material into a 150 ml. beaker andthen adding and mixing in a 50% aqueous glycerine solution until theresulting paste flows readily upon tilting the beaker. The paste thusprepared is poured into the above trough and the brush lowered intocontact with the copper strip. The brush holder is reciprocated 14,400times in about 120 to 123 minutes, so that including the forward andreverse motions of the brush, a total of 28,800 strokes are applied tothe copper strip.

At the end of the foregoing operation, the copper strip is removed,Washed with water, and then with ethyl alcohol. The washed strip is thendried for five minutes at 60 C., cooled in a desiccator, and reweighed.The loss in weight in milligrams represents the polishing power. Thus,if the copper strip loses 10 mg. in weight, the abrasive material beingtested is said to have a polishing power of 10.

The following examples illustrate specific embodiments of the invention:

Example 1 The preparation of stannous pyrophosphate was car- PatentedApr. 3, 1962 ried out by dissolving 1,125 g. of stannous chloridedihydrate in 800 ml. of 19% hydrogen chloride solution at 60 C. To thismixture there was added a solution of 600 g. of sodium acidpyrophosphate (5% excess) and 3,000 ml. of water at 60 C. At thecompletion of the addition, the hydrogen chloride was neutralized by theaddition of sodium carbonate, resulting in the complete precipitation ofthe desired stannous pyrophosphate. The precipitate was then filtered,washed, dried and ground.

The physical and chemical properties of the stannous pyrophosphate wereas follows:

SnO percent 66.8 P205 dO.... Moisture do 0.33 pH do 4.2 Screen analysis:

Cumulative retained 200 do 0.05

Cumulative retained 325 do 2.33 Abrasion value 24.6 Glycerin absorptionvalue 42.0

The above pyrophosphate is particularly desirable in the formulation oftooth paste, because of its high opacifying power. The use of the saidpyrophosphate in a glycerine-containing tooth paste based on insolublesodium metaphosphate is found to yield a product having a dense whitecolor and opacity, making it unnecessary to use any additionalopacifying agent in the formulation.

Stannous pyrophosphate employed as dental polishing agent is also usefulfor its cleansing and stain-removing action. In addition, the stannouspyrophosphate does not produce an unpleasant taste or feel in the month.

In the compounding of tooth pastes it is preferred to employ about 40%to 60% by weight of the dry base, which base may then be combined withvarying percentages of glycerine, gums, water, flavoring materials, etc.In addition, there may be incorporated various wetting, emulsifying orfoaming agents, such'as the sodium alkyl sulfates. In the preparation oftooth powders, it is preferred to employ the above-described stannouspyrophosphate in amount varying from 36%97% by weight of the finalproduct, but a broader range of 197% to include smaller proportions maybe used, particularly if other dentrifrice bases are introduced into theformulation. Other dentrifrice bases which may be employed, togetherwith products made by the present invention are magnesium pyrophosphate,calcium pyrophosphate and insoluble sodium metaphosphate.

The compositions embodying the present invention, when used in adentifrice formulation in paste or powdered form, are capable ofcleaning and polishing the teeth and of restoring and maintaining theirnatural lustre without abrading or scratching the surfaces of the teeth.Moreover, the combination with fluorides provides a novel and eflicientmeans of uniformly supplying controlled amounts of fluoride to the teethwith the ultimate result of preventing and controlling dental caries.

In the preparation of fluorinated dentifrices utilizing theabove-described stannous pyrophosphate the fluoride Example 2 A toothpowder was prepared utilizing the following formulation 'by weight, andalso indicating the range of variation of the components. Thisformulation may also be employed without the fluoride as a generaldentifrice.

Component Weight, Range,

grams percent stannous pyrophosphate 21 1-07 Calcium pyrophosphate.--400 1-07 Sodium'fluoride. 0. 45 0 01-0. 50 Detergent (sodium laurylsulfate) 20 2-8 Saceharln. 1 0. 05-2 Flavoring (essential oil) 4 0. 05-2What is claimed is:

1. A process which comprises bringing together under reactive conditionsstannous chloride and at least a stoichiometric quantity of sodium acidpyrophosphate to effect reaction between said stannous chloride and astoichiometric quantity of said sodium acid pyrophosphate, wherebystannous pyrophosphate is produced.

'2 Process for the preparation of stannous pyrophosphate which comprisesreacting an acidic solution of stannous chloride with a stoichiometricquantity of sodium acid pyrophosphate and thereafter neutralizing thesolution with a basic substance selected from the group consisting ofsodium carbonate and sodium hydroxide, whereby a precipitate of stannouspyrophosphate is obtained.

References Cited in the file of this patent UNITED STATES PATENTS2,174,614 Bornemann et al. Oct. 3, 1939 2,627,493 Merckel et al Feb. 3,1953 2,630,372 Wright Mar. 3, 1953 2,636,808 Hubbard et a1. Apr. 28,1953 2,737,443 Wright Mar. 6, 1956 2,749,278 Moss June 5, 1956 FOREIGNPATENTS 13,199 Great Britain 1894 OTHER REFERENCES H. Beer: Ann. Physik,75,, 152, T. Persos, Ann. Chim. et Phys. 20, 315 (1847).

Ann., 65, 133 and 163 (1848).

Fleitmann and Henneberg: Ann. 65, 387 (1848).

Phosphorus and its Compounds, Van Wazer, vol. 1, Chemistry, IntersciencePublishers, N.Y., 1958, pp. 625 and 628-632,

1. A PROCESS WHICH COMPRISES BRINGING TOGETHER UNDER REACTIVE CONDITIONSSTANNOUS CHLORIDE AND AT LEAST A STOICHIOMETRIC QUANTITY OF SODIUM ACIDPYROPHOSPHATE TO EFFECT REACTION BETWEEN SAID STANNOUS CHLORIDE AND ASTOICHIOMETRIC QUANTITY OF SAID SODIUM ACID PYROPHOSPHATE, WHEREBYSTANNOUS PYROPHOSPHATE IS PRODUCED.